Fabrication of electrically pumped vertical cavity surface emitters employing GaN:Mg/GaN:Ge tunnel junction contacts release_yxgavii7jrb5zfkzgxengzh3ja

Abstract

This thesis investigates novel concepts for the realization of vertical cavity surface emitting lasers (VCSEL) for the blue spectral range. Among them are the p-doping scheme at the p++-contact layer for low contact resistance, different lithographic processing strategies for homogeneous current injection as well as GaN tunnel junctions. Further, conducting AlInN/GaN Bragg mirrors were developed for application in vertical cavity devices. Parameters were first optimized in conventional LED structures with and without additional GaN:Ge tunnel junction layers to be then applied in VCSEL structures. For p-type doping in MOVPE, improved conditions were determined for contact formation to p-GaN layers and for tunneling contacts. The best scheme as applied to LEDs had to be modified for VCSEL structures with GaN tunnel junctions in order to allow an efficient acceptor activation and to avoid surface roughening. GaN tunnel junctions were also compared to an established ITO-layer as a current spreading layer on top of p-doped GaN:Mg. ITO layers absorb significantly more light than the GaN based tunnel junction. For p-side current injection, current confinement schemes based on a lateral injection, overgrown TJ structures and selectively deposited ITO layers were compared. In these approaches, the spontaneous light emission intensity increased up to 10 times for TJ-GaN VCSEL structures as compared to a reference ITO-VCSEL. Injection current densities of up to 30 kA/cm2 were demonstrated even with tunnel junctions at moderate bias. Yet, lasing could not be achieved but the relevant issues are addressed in this work. Besides further improvements on current injection by better acceptor activation techniques, an improved interface quality of the dielectric DBR forming the upper mirror is required. Moreover, the reproducibility of the InGaN/GaN active region must be revised in order to maximize the internal quantum efficiency. A concept of conducting Bragg mirrors was investigated in order to enable electron injection into the [...]
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